1. Field of the Invention
The present invention relates to a method of transferring Bloch lines, the method being capable of being used in a magnetic memory for recording/reproducing information by using Bloch lines formed as a carrier of unit information in a domain wall of a stripe domain.
The present invention also relates to a magnetic memory for performing write/read access of information by transferring Bloch lines by a Bloch line transfer method different from a prior art method.
A magnetic memory of this type may be used as a versatile memory in a variety of applications for various electronic devices since information can be recorded at a very high density.
2. Related Background Art
Various devices such as magnetic tapes, Winchester disks, floppy disks, optical disks, optomagnetic disks, and magnetic bubble memories have been used as memories such as external computer memories, electronic file memories, and still image file memories. These memory devices, excluding the magnetic bubble memory, require movement of a write/ read head relative to the memory when information is to be written or read. During relative movement of such a head, a data train can be permanently recorded in a data track, or the data train permanently written in the data track is read.
A higher recording density requires complicated tracking control for accurately-tracking the head along the data track. Imperfect control degrades the quality of the read/write signal. The quality of the read/write signal is also degraded by vibrations of a head carrier mechanism and dust or the like attached to the surface of the memory. In addition, in a memory device such as a magnetic tape wherein a head is brought into sliding contact with the surface of the magnetic tape to perform read/write access, wear of the memory device occurs. In a memory device such as an optical disk wherein read/ write access is performed while a head is separated from the memory device, focusing control and tracking control are inevitable. If such control operations are not accurately performed, the quality of the read/write signal is undesirably degraded.
As described in U.S. Ser. No. 801,401 filed Nov. 25, 1985 (assigned to the same assignee as is the present invention), a magnetic bubble memory can record data at a predetermined position and can transfer data. In addition, data can be reproduced at a predetermined position while data is being transferred. Movement of the head relative to the memory is not required. Therefore, a magnetic bubble memory can achieve high operational reliability without problems posed by other memories when a recording density is to be increased.
In a magnetic bubble memory, however, a magnetic field is applied to a thin magnetic film (e.g., a magnetic garnet film) having an axis of easy magnetization in a direction perpendicular to the surface of the magnetic film to form a circular domain (i.e., a bubble), and the bubble so formed is used as a data bit. Even if a minimum size bubble (a diameter of 0.3 .mu.m), whose size is limited by the material properties of the currently available garnet film, is used, the maximum recording density is several tens of Mbits/cm.sup.2. A higher recording density cannot be expected.
A Bloch line memory see, (e.g., U.S. Pat. No. 4,583,200) has received a great deal of attention as a memory having a higher recording density than that of the above-mentioned magnetic bubble memory. A Bloch line recording/reproducing method is proposed in U.S. Ser. No. 800,770 filed Nov. 25, 1985. In the Bloch line memory, a Neel domain wall structure (Bloch line) pair sandwiched between the Bloch domain wall structures located around the domain formed by the thin magnetic film is used as a data bit. Therefore, the recording density of the Bloch line memory can be increased to about 100 times that of the magnetic bubble memory. For example, if a garnet film having a bubble diameter of 0.5 .mu.m is used, the recording density can be as high as 1.6 Gbits/cm.sup.2.
FIG. 1 is a perspective view showing a magnetic structure constituting a Bloch line memory described in the above U.S. patent.
Referring to FIG. 1, a thin magnetic garnet film 4 is formed on a nonmagnetic garnet substrate 2 made of GGG, NdGG or the like. The garnet film 4 can be formed by liquid-phase epitaxy (LPE) and has a thickness of about 5 .mu.m. A stripe-like magnetic domain (to be referred to as a stripe domain hereinafter) 6 is formed in the thin magnetic garnet film 4. A domain wall 8 is formed as a boundary region of the stripe domain 6. The width of the stripe domain 6 is about 5 .mu.m, and its length is about 100 .mu.m. The thickness of the domain wall 8 is about 0.5 .mu.m. As indicated by arrows A and B in FIG. 1, the direction of magnetization within the domain 6 is upward (arrow A); and the direction of magnetization outside the domain 6 is downward (arrow B).
The magnetization within the domain wall 8 is twisted from the inside to the outside (i.e., the plane on the side of the stripe domain 6), i.e., from the arrow A to the arrow B. The direction of twist inside Bloch lines 10 is opposite to that outside the Bloch lines in the domain wall 8. Referring to FIG. 1, the direction of magnetization at the central portion along the direction of thickness of the domain wall 8 is indicated by arrow C. The direction of magnetization of the Bloch lines 10 is indicated by arrow D.
An external downward bias magnetic field H.sub.B is applied by a permanent magnet or the like to the magnetic structure including the stripe domain 6.
As shown in FIG. 1, the Bloch lines 10 formed in the domain wall 8 include two kinds of lines having opposite directions of magnetization. Two such lines, one of each of the kinds of Bloch lines, constitute a Bloch line pair. The presence and absence of the Bloch line pair correspond to data "1" and data "0", respectively. The Bloch line pairs are regularly located in the domain wall 8. In other words, each Bloch line pair is located in one of a plurality of potential wells (to be described later). When a pulsed magnetic field having a direction perpendicular to the surface of the substrate is applied to the Bloch line, the Bloch line is continuously transferred to the next potential well. Recording (writing of the Bloch line pair in the domain wall 8) of information in the Bloch line memory and reproduction (readout of the Bloch line pair from the domain wall 8) of information from the Bloch line memory are performed at predetermined positions while the Bloch line pair is transferred within the domain wall 8. In the recording and reproduction modes, a pulsed magnetic field having a predetermined intensity and a direction perpendicular to the surface of the substrate can be applied to the predetermined positions. As is omitted from FIG. 1, pulse energization conductive patterns as a pulsed magnetic field applying means are formed on the surface of the thin magnetic film 4 at predetermined positions respectively corresponding to the stripe domains 6.
In the Bloch line memory described-above, the potential wells for the Bloch line pairs are constituted by forming regular Bloch line stabilizing patterns on the surface of the thin magnetic film so as to cross the domain wall.
FIG. 2 is a plan view of part of the Bloch line memory, showing the stabilizing patterns.
Referring to FIG. 2, a large number of parallel line conductive patterns 9 are formed on the surface of the thin magnetic film 4 so as to cross the stripe domain 6. The patterns 9 comprises conductive layers made of Cr, Al, Au, Ti, or the like. The width of each pattern 9 is about 0.5 .mu.m, and the patterns 9 are aligned at pitches of about 1 .mu.m. The patterns 9 are formed to generate magnetic distortion to form potential wells. When the patterns 9 are formed regularly, the alignment of the potential wells within the domain wall 8 can be regular and cyclic. Each pattern 9 may comprise a magnetic layer or a structure obtained by implanting ions (e.g., H, He, or Ne ions) near the surface of the thin magnetic film 4, in addition to the conductive layer described above. The potential wells formed by these patterns are symmetrical with respect to the Bloch line transfer direction.
As described in U.S. Pat. No. 4,583,200, Bloch line transfer can be performed as follows. A pulsed magnetic field having a direction perpendicular to the surface of the thin magnetic film 4 is applied thereto, and precession action of magnetization constituting the Bloch lines is utilized to shift the Bloch line to the adjacent potential well. However, when symmetrical potential wells are used and a magnetic field having simple rectangular pulses is used as the pulsed magnetic field, the Bloch lines cannot be stably shifted in a specific direction. For this reason, as shown in FIG. 3, the fall time of the Bloch line transfer pulsed magnetic field Hp is sufficiently longer than the rise time thereof, thereby preventing reversible transfer of the Bloch lines in the specific direction.
An electric circuit for generating the pulsed magnetic field is complicated. In addition, since the fall time is longer than the rise time, the transfer speed cannot be increased, resulting in inconvenience.